1. Field of the Invention
The present invention relates to a toner conveying apparatus, and more particularly, to a toner conveying apparatus which supplies toner to a developing apparatus which develops an electrostatic image formed on an image bearing member in an image forming apparatus using toner.
2. Description of the Related Art
In a conventional image forming apparatus, if an electrostatic image on an image bearing member is developed and an amount of toner in a developing apparatus becomes less than a defined amount, toner is supplied into the developing apparatus from a toner supply apparatus (hopper, hereinafter) which stores toner. An apparatus which uses one component developer and an apparatus which uses two component developer supply the toner in the same manner.
A hopper normally includes a toner cartridge which supplies new toner, and a buffer portion which accommodates toner supplied from the toner cartridge (see Japanese Patent Application Laid-open No. 3-217879 for example). If the amount of toner in the buffer portion of the hopper is reduced, a toner sensor provided in the buffer portion detects the shortage of toner, toner is supplied from the toner cartridge, and the toner amount in the buffer portion is always maintained at a constant level.
A conventional hopper has a long shape as shown in FIG. 1 in many cases. In a hopper 200 of this type, toner supplied from a toner cartridge 201 drops into a buffer portion 202 by gravity and stays therein, and the toner is supplied to a developing apparatus 204 by a screw 203 provided on a bottom of the buffer portion 202. In the case of a system replenishing the buffer portion 202 with toner by gravity, bulk density of toner in the buffer portion 202 is prone to be varied while the toner stays in the buffer portion 202 and the amount of toner to be fed to the developing apparatus 204 is prone to be varied.
As one means for reducing the variation in bulk density of toner in the hopper, there is a system for circulating toner in a shallow buffer portion as shown in FIG. 12. According to the hopper of this system, toner supplied into the buffer portion 211 from the toner cartridge is circulated and conveyed into the buffer portion 211 by stirring screws 212a and 212b and the toner is equalized. Toner conveyed to an opening 213 by a first conveying screw 212c in the buffer portion 211 is conveyed to a developing apparatus by a second conveying screw in a toner conveying section (not shown). With this, as compared with a system which replenishes the buffer portion with toner by gravity, it is possible to suppress the bulk density variation in the buffer portion.
Further, in order to further stabilize the bulk density, there is a method in which toner is crammed into the toner conveying section provided with the second conveying screw, thereby eliminating gaps in the toner conveying section, and variation in the amount of toner to be fed to the developing apparatus is reduced. More specifically, a relation between toner feeding amounts by the two conveying screws is established such that a toner feeding amount of the first conveying screw is equal to or greater than a toner feeding amount of second conveying screw, so as to prevent gaps from being formed in the toner conveying section during conveying of toner by the second conveying screw.
According to the structure of the conventional hopper, however, as the length of the second conveying screw in the toner conveying direction becomes longer, greater rotation torque for sending the toner crammed into the toner conveying section is required, and a pressure applied to the toner itself is increased. If temperature influences of the image forming apparatus and environment are added thereto, the toner concentration degree is increased, toner adheres to blades of the second conveying screw, toner is not fed in the conveying direction and toner rotates together with the blades and the toner is not conveyed. If such a phenomenon occurs, if the relation that the toner feeding amount of the first conveying screw is equal to or greater than the toner feeding amount of the second conveying screw is established, toner which is not conveyed is further crammed into a certain portion, the pressure partially applied to toner is further increased. With this, a vicious circle that the toner concentration degree is increased and toner adheres to the blades of the conveying screw and the pressure applied to the toner is further increased is repeated. As a result, the rotation of the conveying screw is locked, supply failure of toner occurs, toner clump is generated and there is an adverse possibility that image failure occurs.